(1) Field of the Invention
This invention relates to write-once optical recording media, and more specifically to optical recording media useful as external memories for computers and for recording various information such as video and audio information as well as to a method for recording information on the media and reading them out from the media.
(2) Description of the Prior Art
As write-once optical recording media of the above-described sort, there have been proposed recording media having inorganic recording layers of thin films of low melting-point metals such as tellurium, tellurium alloys and bismuth alloys, and as disclosed in U.S. Pat. No. 4,298,975 for example, recording media making use of phthalocyanine dyes as recording layers.
These recording media are however accompanied by a problem that their productivities are all low, since their recording layers must be formed in vacuo by vacuum evaporation, sputtering or the like. Furthermore, media having inorganic recording layers are limited in recording density due to the greater thermal conductivities of the recording layers. There is also a potential danger with respect to toxicity since they employ poisonous materials such as tellurium. On the other hand, optical recording media containing phthalocyanine dyes as recording layers usually require a shifting treatment in which recording layers obtained generally by vacuum evaporation are exposed to heat or vapor of an organic solvent, because the optical characteristics of the recording layers are not sensitive to the oscillation wavelengths of semiconductor lasers. This shifting treatment is cumbersome and takes as long as 1-72 hours. Therefore, these phthalocyanine-containing optical recording media have not yet found actual utility.
With a view toward providing a solution to the above-mentioned problems, optical recording media with recording layers formed by coating soluble organic dyes have been proposed. For example, processes making use of the spin coating technique have been developed to coat organic dyes which are soluble in an organic solvent and exhibit absorption in the oscillation wavelength range of semiconductor lasers, such as dithiol metal complexes, polymethine dyes, squarylium dyes and naphthoquinone dyes. Some of these processes have already been practiced. Such conventional recording media however had poor durability as demonstrated by those containing, for example, polymethine or squarylium dyes as recording layers out of the dyes proposed to date or required additionally reflective thin layers made of inorganic materials such as metal or metal oxide, because they had very low reflectance when provided solely with recording layers of dyes such as dithiol metal complexes out of the dyes proposed to date.
For example, U.S. Pat. No. 4,492,750 relates to media which use alkyl-substituted naphthalocyanine dyes. It discloses an optical recording medium having (1) a reflective layer of a material such as Al provided on a glass or polymethyl methacrylate substrate; and (2) a layer of an optical recording composition provided on the reflective layer and containing vapor-treated particles of an alkyl-substituted naphthalocyanine dye, the particle sizes of which range from 0.005 .mu.m to 0.1 .mu.m, dispersed in a polymeric binder. As disclosed in the above U.S. patent, the optical recording layer cannot be formed directly on the substrate and the reflective layer made of an inorganic material such as Al must be formed additionally on the substrate by a vacuum process such as vacuum evaporation. The fabrication process of the optical recording medium is thus rather complicated. In addition, the above optical recording medium is accompanied by a more serious problem. A recording layer making use of an organic dye has an inherent feature, that is, a low thermal conductivity. Hence, it is potentially expected to exhibit high recording sensitivity. When a reflective layer made of a high thermal-conductivity metal or inorganic material is provided, the thermal energy produced by a writing laser beam irradiated onto the recording layer is however caused to dissipate through the reflective metal layer due to the high thermal conductivity of the reflective metal layer, so that the thermal energy is not effectively used for the formation of pits or holes (which correspond to signals). As a result, the recording sensitivity is reduced to a considerable extent. Let's now assume that a reflective layer made of an inorganic material such as Al is provided. When a laser beam is irradiated through the substrate for recording signals or reading them out, the laser beam is obviously not allowed to reach the recording layer even if the substrate per se is transparent. This is obvious because the laser beam is shut off by the reflective layer of the inorganic material which practically prevents transmission of light therethrough. Whenever such a reflective layer is provided, it is naturally impossible to perform the recording and reading-out of signals through the associated substrate. Accordingly, the recording and reading-out of signals have to be conducted on the side of the recording layer. In this case, slightest existence of dust or scratches on the surface of the recording layer results in considerable disturbance to the accurate recording and reading-out of signals which take the form of pits or holes. For practical application, the above-mentioned optical recording medium thus requires a dust protective layer as an overcoat on the recording layer. If it becomes feasible to conduct the recording and reading-out of signals by means of a laser beam through a transparent substrate, such a dust protective layer will not be required at all. Because the existence of dust or scratches on the medium surface on the incident side of the laser beam, where the laser beam is still un-focused, has no effect on recording and reading-out of the signals. As mentioned above, optical recording media with reflective layers made of inorganic (metallic) materials such as Al are accompanied by numerous drawbacks. It has thus been desired to develop an optical recording medium which permits recording and reading-out of signals without need for a reflective layer made of an inorganic material and has a highly-durable recording layer formed by coating an organic dye.